At the present time proteinaceous polymers such as casein and soy protein are used in various coating compositions; paper coatings and paints and the like. These proteinaceous materials, and particularly casein, have probably been the most generally used colloidal materials in the coating industry, especially for high grade paper coatings. Dispersions of casein are readily prepared and are good as pigment dispersion media. Moreover, they are compatible with many resin and elastomeric latices and the casein is easily insolubilized.
Pigmented coatings are generally applied to various substrates for several reasons, i.e., for protection of the substrate, for decorative purposes, and for making the substrate more suitable for other operations such as printing or laminating.
While clearly suitable, casein and soy protein are susceptible to bacterial degradation and more particularly, these proteinaceous materials are subjected to very extensive variations in terms of cost and availability since they are used as food proteins and hence often become too expensive or unavailable for usage in coatig as discussed above.
Efforts to substitute other materials for these proteinaceous polymers have not been successful either because the materials used do not give the desired coating properties or dispersing properties of the proteinaceous materials, or because they themselves are incapable of having stable viscosities at high and medium solids concentrations, or fail to give the proper adhesive strength and pigment binding power. In short, they fail to give the desired rheological properties obtained when casein is used.